Seed and Seedholder Combinations for High Quality Growth of Large Silicon Carbide single Crystals

ABSTRACT

A silicon carbide seeded sublimation method is disclosed. The method includes the steps of nucleating growth on a seed crystal growth face that is between about 1° and 10° off-axis from the (0001) plane of the seed crystal while establishing a thermal gradient between the seed crystal and a source composition that is substantially perpendicular to the basal plane of the off-axis crystal.

RELATED APPLICATIONS

This is a divisional of Ser. No. 10/915,095 filed Aug. 10, 2004.

BACKGROUND

The present invention relates to the sublimation growth of large singlepolytype crystals of silicon carbide.

Silicon carbide (SiC) has been known for many years to have excellentphysical and electronic properties which theoretically allow productionof electronic devices that can operate at higher temperatures, higherpower and higher frequency than devices produced from silicon or galliumarsenide. The high electric breakdown field of about 4×106 V/cm, highsaturated electron drift velocity of about 2.0×107 cm/sec and highthermal conductivity of about 4.9 W/cm-K make SiC conceptually suitablefor high frequency, high power applications.

Silicon carbide also has an extremely wide band gap (e.g., 3 electronvolts (eV) for alpha SiC at 300K as compared to 1.12 eV for Si and 1.42for GaAs), has a high electron mobility, is physically very hard, andhas outstanding thermal stability, particularly as compared to othersemiconductor materials. For example, silicon has a melting point of1415° C. (GaAs is 1238° C.), while silicon carbide typically will notbegin to disassociate in significant amounts until temperatures reach atleast about 2000° C. As another factor, silicon carbide can be fashionedeither as a semiconducting material or a semi-insulating material.

As obstacles to its commercialization, however, silicon carbide requireshigh process temperatures for otherwise ordinary techniques, goodstarting materials are difficult to obtain, certain doping techniqueshave traditionally been difficult to accomplish, and perhaps mostimportantly, silicon carbide crystallizes in over 150 polytypes, many ofwhich are separated by very small thermodynamic differences.Nevertheless, recent advances, including those discussed in U.S. Pat.Nos. 4,865,685 and 4,866,005 (now Re. 34,861) have made it possible toproduce silicon carbide and silicon carbide based devices on acommercial basis and scale.

One of these advances has been the use of “off-axis” growth techniquesto produce single (i.e., single polytype) crystal epitaxial layers andbulk single crystals. Generally speaking, the term “off-axis” is used todescribe crystal growth techniques in which the seed crystal ispresented for growth with its surface cut at an angle (usually betweenabout 1 and 10 degrees) away from a basal plane or a major axis.Basically, it is accepted that an off-axis growth surface presents alarge number of opportunities for step (lateral-step) growth that iscontrolled by the polytype of the crystal. The off-axis preparation ofthe substrate surfaces causes a series of steps and ledges to be formedto accommodate the (intentional) misorientations. The average spacingand height of the steps are largely determined by the degree ofmisorientation; i.e. the selected off-axis angle.

Alternatively, on-axis growth has been generally disfavored because anon-axis growth surface defines and presents far fewer ordered growthsites, and because the (0001) plane of silicon carbide does not containpolytype information. Instead, on-axis growth must proceed from defectsand is thus less easily controlled.

Off-axis growth, however, presents its own unique challenges. Inparticular, bulk growth of large silicon carbide crystals (e.g. thosesuitable for wafer and device substrates) is generally carried out inseeded sublimation growth techniques, the details of which are laid outin the U.S. Pat. No. 4,866,005 incorporated above, and which have becomefamiliar to those of skill in this art. Another relevant summary is setforth in Zetterling, Process Technology for Silicon Carbide Devices,INSPEC (2002) §§ 2.2.2.1-2.2.2.4.

In brief summary, a seeded sublimation technique includes a graphite (orsimilar) crucible that holds the source powder and the seed crystal. Thecrucible is heated (e.g. inductively) in a manner that establishes athermal gradient between the source powder (hotter) and the seed crystal(cooler). The main gradient is typically referred to as the “axial”gradient because it typically (and intentionally) falls parallel to theaxis of the growth crucible in which the source materials are at thebottom or lower portions and the seed crystal is attached to the upperportions and opposite some or all of the source materials. When thesource powder is heated above SiC's sublimation temperature, the powdergenerates vaporized species that migrate generally or predominantlyalong the axial gradient and condense upon the seed crystal to producethe desired growth. In typical SiC seeded sublimation techniques, thesource powder is heated to above about 2000° C.

During seeded sublimation growth of bulk single crystals, in order tokeep the basal plane of the crystal (the plane that is parallel to theprincipal plane of symmetry of the crystal) on the growth surface of thecrystal, off-axis techniques force the growth of convex crystals. Inturn, encouraging such convex growth in a seeded sublimation techniquetypically requires the application of relatively high radial thermalgradients in addition to the axial (growth direction) thermal gradient.These radial gradients add stress to the growing crystal at growthtemperatures, and these in turn create other stresses when the crystalcools to use or room temperature.

In silicon carbide, off-axis growth presents yet another problem basedon the combination of two factors. As the first factor, in siliconcarbide, the primary slip plane is the basal plane. As the secondfactor, changes in thermal gradients apply stress to crystals in thedirection in which the gradient is changing; i.e., changes in the axialgradient apply stress to the crystal in the axial direction. As isunderstood by those familiar with such factors, the existence of thermalgradients does not generate the stress, but rather the rate of change ofthose thermal gradients. Stated differently, if a thermal gradientrepresents the change in temperature per unit of distance, the stressesare generated by the rate of change of the gradient per unit distance(i.e., a second derivative function).

When—as in off-axis sublimation growth—the basal plane is off axis tothe major growth direction, these axial forces (caused by the rate ofchange of the gradient) include and thus apply a component parallel tothe slip plane and thus generate and encourage slip defects. Themagnitude of the basal-parallel component increases as the off axisangle increases in a manner common to the well-understood resolution ofvectors into their respective components.

As a result, increasing the angle of the off-axis presentation of agrowing silicon carbide crystal, or its seed, increases the slip forcesapplied to the basal plane and these forces in turn increase theconcentration of slip defects per unit area.

By comparison, in on-axis growth the axial forces caused by changes inthermal gradients neither include nor apply a component parallel to thebasal (slip) plane. Thus, slip defects can be more easily avoided duringon-axis growth. As noted above, however, on-axis growth presents lesspolytype information and lower surface step density per unit area, thusmaking on-axis growth a relatively difficult technique for replicatingthe polytype of the seed crystal into the growing crystal.

Accordingly, both on and off-axis seeded sublimation growth techniquesfor silicon carbide present particular disadvantages.

SUMMARY

In a first aspect, the invention is a silicon carbide growth systemcomprising a crucible; a silicon carbide source composition in saidcrucible; a seed holder in said crucible; a silicon carbide seed crystalon said seed holder; means for creating a major thermal gradient in saidcrucible that defines a major growth direction between said sourcecomposition and said seed crystal for encouraging vapor transport fromsaid source composition to said seed crystal; and said seed crystalbeing positioned on said seed holder with the macroscopic growth surfaceof said seed crystal forming an angle of between about 70° and 89.5°degrees relative to said major thermal gradient.

In a similar aspect, the invention is a silicon carbide growth systemcomprising a crucible; a silicon carbide source composition in saidcrucible; a seed holder in said crucible; a silicon carbide seed crystalon said seed holder; means for creating a major thermal gradient in saidcrucible that defines a major growth direction between said sourcecomposition and said seed crystal for encouraging vapor transportbetween said source composition and said seed crystal; and said seedcrystal being positioned on said seed holder with the macroscopic growthsurface of said seed crystal forming an angle of between about 70° and89.5° degrees relative to said major thermal gradient and said majorgrowth direction and with the crystallographic orientation of said seedcrystal having the c-axis of said crystal forming an angle with saidmajor thermal gradient of between about 0° and 2°.

In another aspect, the invention is a method of growing large singlecrystals of silicon carbide with reduced defects. The method comprisesnucleating growth on a seed crystal growth face that is between about0.5° and 10° off-axis from the (0001) plane of the seed crystal whileestablishing a thermal gradient between the seed crystal and a sourcecomposition that is substantially perpendicular to the basal plane ofthe off-axis crystal.

In another method aspect, the invention comprises establishing a majorgrowth temperature gradient between a silicon carbide seed crystal and asilicon carbide source composition and positioning the seed crystal onthe seed holder with the macroscopic growth face of the seed crystal atan angle of between about 70° and 89.5° with respect to the major growthgradient.

In yet another method aspect, the invention comprises establishing amajor growth temperature gradient between a silicon carbide seed crystaland a silicon carbide source composition; presenting a growth face onthe seed crystal that is between about 1° and 10° off axis from the(0001) plane; and positioning the seed crystal on the seed holder withthe growth face at an angle of between about 70° and 89.5° with respectto the major growth gradient.

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe followed detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic diagram of a seeded siliconcarbide system according to the present invention.

FIG. 2 is a diagram illustrating the geometric relationships between andamong the seed crystal and the major thermal gradient according to thepresent invention.

FIG. 3 is a schematic diagram of an off-axis seed in an off-axisorientation.

FIG. 4 is a schematic diagram of an on-axis seed in an off-axisorientation.

FIG. 5 is a diagram illustrating the geometric relationships between anoff-axis seed crystal and a major thermal gradient.

FIG. 6 is a vector diagram illustrating stress applied to the basal(slip) plane of off-axis silicon carbide.

FIG. 7 is a vector diagram illustrating stress applied to the basal(slip) plane of on-axis silicon carbide.

DETAILED DESCRIPTION

In a first embodiment, the present invention is a seeded sublimationgrowth system for silicon carbide. In this aspect, the inventioncomprises a crucible, a silicon carbide source composition in thecrucible, a seed holder in the crucible, a silicon carbide seed crystalon the seed holder, and means for creating a major thermal gradient inthe crucible that defines a major growth direction between the sourcecomposition and the seed crystal for encouraging vapor transport betweenthe source composition and the seed crystal. The seed crystal ispositioned on the seed holder with the macroscopic growth surface of theseed crystal forming an angle of between about 70° and 89.5° relative tothe major thermal gradient and the major growth direction and with thecrystallographic orientation of the seed crystal having the c-axis ofthe crystal forming an angle with the major thermal gradient of betweenabout 0° and 2°.

FIG. 1 is a schematic cross sectional view of a representative seededsilicon carbide sublimation system according to the present invention.Those of ordinary skill in this art will recognize that FIG. 1 is ageneralized view of such a system, is not to scale, and that a number ofvariations can be included or substituted while still practicing theinvention claimed herein. Accordingly, FIG. 1 should be understood asillustrative and exemplary rather than limiting of the presentinvention.

FIG. 1 illustrates a crucible broadly designated at 10 that is formed ofa tubular portion 11, a bottom portion 12, and a seed holder 13. In manyembodiments, the preferred material for the crucible 10 is graphite, butother materials can be incorporated either partially or entirely, andpotentially as coatings. Examples include tantalum carbide (TaC) niobiumcarbide (NbC) and related carbide compositions.

A silicon carbide source composition 14 is positioned in the crucible.As illustrated in FIG. 1, portions of the silicon carbide sourcecomposition 14, which is most typically a silicon carbide source powder,are positioned in the crucible 10.

A seed crystal 16 is positioned on the seed holder 13. In order toencourage sublimation growth on the seed crystal 16, a thermal gradientrepresented by the arrow 17 is established in the crucible 10. Asillustrated in FIG. 1 and as frequently carried out in these techniques,the graphite or other crucible 10 is heated inductively by passingcurrent through a coil illustrated in cross-section at 20. The coil istypically maintained outside of a quartz tube 21. In order to create athermal gradient, insulation schematically illustrated at 22 ispositioned in and around the quartz tube 21 and the crucible 10. Thegoal is to create a gradient in which the source powder is heated toapproach the sublimation temperature of silicon carbide while the seedcrystal 16 is maintained at a slightly lower temperature, with thechanging temperature being relatively even along the gradient 17 in mostcircumstances. The temperatures of the source material 14 and the seedcrystal 16 are preferably monitored, typically with infrared (optical)techniques and instruments. If necessary the current applied to the coil20 or the position of the insulation 22 can be adjusted to provide thedesired or necessary gradient. Alternatively, some techniques andsystems use resistance heating rather than induction heating.

Typically, sublimation growth is carried out in an atmosphere that isotherwise inert to the sublimation process. The usual options include anoble gas such as argon, other gases such as hydrogen, or even a vacuum.

The phrase “major thermal gradient” is used herein to highlight theunderstanding in this art that at the extremely high temperatures beingused, a number of thermal gradients can exist extending in a numberdirections. Thus, the gradients are properly understood as vectorquantities; i.e., being characterized by both magnitude and direction.The major thermal gradient, however, is the one established between theseed crystal 16 and the source powder 14, and is best understoodconceptually as being a vertical (or “axial”) gradient in the schematicorientation of FIG. 1 and in most growth techniques. Because of thenature of seeded sublimation growth, such growth will predominantlyproceed in the same direction as the major thermal gradient.

Other growth techniques specifically incorporate radial temperaturegradients; i.e., those perpendicular to the major thermal gradient andsmaller in magnitude than the major thermal gradient. Indeed, if aradial gradient exceeds the magnitude of an axial gradient, the mostgrowth will occur along the radial gradient rather than the axialgradient. Thus, radial gradients are sometimes created and usedintentionally and the invention can be used in conjunction with suchradial gradients if desired or necessary for any particular goal orpurpose.

More fundamental to the invention, however, is the positioning of theseed crystal 16 on the seed holder 13. In the invention, the seedcrystal 16 is positioned on the seed holder 13 with the microscopicgrowth surface of the seed crystal forming an angle of between about 70and 89.5° (FIG. 1 is not necessarily to scale in this regard) relativeto the major thermal gradient 17 and the major growth direction that isdefined by and thus parallel to the major thermal gradient. Thecrystallographic orientation of the seed crystal is also positioned sothat the c-axis of the crystal 16 forms an angle with the major thermalgradient 17 of between about zero and 2°.

In the preferred embodiments of the invention, the growth surface of theseed crystal 16 is between about 1° and 10° off-axis from the (0001)plane of the crystal. Stated differently, in this embodiment, both theseed holder 13 and the seed crystal 16 are off-axis.

Alternatively, advantages can exist in embodiments in which the growthsurface of the seed crystal is a basal plane of the crystal and whensuch basal plane is the (0001) plane of the crystal. Stated differently,in this embodiment, the seed holder 13 is off-axis while the seedcrystal 16 is on-axis. In the more preferred embodiments, however, theseed crystal itself is off axis so that the c-axis of the crystal formsan angle with the major thermal gradient of less than about 1°.

As further well understood in this art, the c-axis of the crystal is theaxis perpendicular to the principal plane of symmetry. Additionally, andas also used consistently herein, the plane of a crystal that isparallel to the principal plane of symmetry is referred to as a basalplane.

A number of aspects of the invention are illustrated schematically inFIGS. 2 through 7. As a brief prologue, these figures illustrate severalrelationships. One such relationship is that of the seed crystal face tothe seed crystal c-axis. A face that is non-perpendicular to the c-axisis an “off-axis” face. A second relationship is that of the seed holderto the major thermal gradient in the growth environment. When the seedholder is non-perpendicular to the major thermal gradient, the seedholder is “off-axis” to that gradient. A third relationship is thatbetween the face (macroscopic growth surface) of the seed crystal andthe major thermal gradient when the seed crystal is mounted for growthon the seed holder.

In FIG. 2, the thermal gradient is again schematically illustrated at 17and the seed crystal at 16. The growth surface of the seed crystal isdesignated at 23 and the angle alpha (a) is illustrated between themajor thermal gradient 17 and the growth surface 23. As noted above,alpha is preferably between about 70° and 89.5°, with a range of betweenabout 80° and 88° being more preferred. The c-axis of the crystal isillustrated in 24, and is separated from the major thermal gradient 17by a relatively small angle beta (β) of between about 0° and 2°. BecauseFIG. 2 is schematic, it will be understood that alpha and beta are notdrawn to scale.

FIG. 3 is another schematic illustration showing the embodiment in whichboth the seed holder 13 and the seed crystal 16 are in an off-axisorientation. As seen in this combination, the c-axis of the crystal 24and the major thermal gradient 17 are essentially parallel to each otherwhile the growth surface 23 presents appropriate growth stepopportunities in a manner desired or advantageously presented by usingan off-axis seed. FIG. 3 should be viewed in comparison to FIGS. 5, 6and 7 as helping to further illustrate the invention.

FIG. 4 illustrates that certain embodiments of the invention provideadvantages even with on-axis seed crystals. As shown in FIG. 4, the seedholder 13 provides an off-axis orientation for the on-axis seed crystal16. Because the crystal will tend to grow with its basal planeperpendicular to the growth direction, many of the propagating defectssuch as 1c screw threading dislocations will intersect with each other,an effect referred to as recombination (not to be confused with the“recombination” of electrons and holes in p-n devices). In turn suchrecombination reduces the number of defects in the resulting crystal. Inconventional off-axis growth, the basal plane is not perpendicular tothe growth direction and thus off-axis growth presents fewerrecombination opportunities.

FIG. 5 illustrates some of the relationships in a more conventional(prior art) off-axis technique in which the growth face 23 of thecrystal 16 is an off-axis face, but with the c-axis of the crystal 24being off-axis from the major thermal gradient 17 to the same extent asthe crystal is off-axis from a basal plane 18. As noted in theBackground, this presents some favorable aspects for replicating thepolytype of the seed crystal 16, but creates the situation shown in FIG.6. FIG. 6 is a vector diagram in which the growth surface 23 of the seedcrystal (not otherwise illustrated in FIG. 6) is off-axis to the majorthermal gradient 17. Any changes in the thermal gradient are exerted inthe same vector direction as the thermal gradient 17. The vectorrepresenting the forces from the changing gradient is illustrated at 25.Because the vector 25 is not directly perpendicular to the growthsurface 23, its application against surface 23 can be resolved into twocomponent vectors 26 and 27. In particular, vector component 26 exerts aforce parallel to the basal plane. As further noted in the Background,because the basal plane is the slip plane for silicon carbide, thevector component 26 encourages slip defects during growth.

FIG. 7 is a vector diagram illustrating the advantages of the invention.Using the invention, because the c-axis of the crystal is eitherparallel or nearly parallel to the thermal gradient 17, the force vector25 generated by the changes in the thermal gradient is appliedsubstantially entirely perpendicularly against the slip plane 23. As aresult, there is either no component, or at most a minimal component, offorce exerted against the slip plane. Thus, the invention helps avoidslip defects during seeded sublimation growth.

Theoretically, the invention can include the use of a on-axis seedholder with a wedge-shaped seed in order to present both an off-axisgrowth face and a basal plane perpendicular to the growth direction(alternatively stated as the c-axis parallel to the growth direction).Such a wedge shaped seed presents a number of mechanical difficulties,however, and thus will be used less frequently. Similarly, it will beunderstood that tilting the entire crucible or tilting the thermalgradient can produce the same effective geometry. Such techniques arerelatively complex, however, and thus may be less attractive from apractical or commercial standpoint, all other factors being equal.

In preferred embodiments the seed crystal has a polytype selected fromthe group consisting of the 4H and 6H polytypes of silicon carbide.

In preferred embodiments, the seed-holding face of the seed holder ismaintained at an angle that is between about 4° off horizontal(horizontal being defined as perpendicular to the major growth gradient)and with the growth face of the seed crystal about 4° off-axis from the(0001) plane of the crystal. This can, of course (and like all of theother angles described herein), be expressed in terms of a complementaryangle; i.e., and angle of about 86° with respect to the major growthgradient.

Accordingly, in another aspect, the invention is a method of growinglarge single crystals of silicon carbide with reduced defects.

In its broadest aspect, the method comprises nucleating growth on a seedcrystal growth face that is between about 1° and 10° off-axis from the(0001) plane of the seed crystal while establishing a thermal gradientbetween the seed crystal and a source composition that is substantiallyperpendicular to the basal plane of the off-axis crystal.

Further to this aspect, the invention comprises establishing the majorgrowth temperature gradient between the silicon carbide seed crystal andthe silicon carbide source composition, presenting a growth face on theseed crystal that is between about 1° and 10° off-axis from the (0001)plane; and positioning the seed crystal on the seed holder with thegrowth face at an angle of between about 70° and 80° C. with respect tothe major growth gradient.

Accordingly, the combination of an off-angle seed holder with anoff-angle seed crystal results in a seed surface that is microscopicallyon-axis (i.e., the crystallographic c-axis of the silicon carbide seedwafer forms an angle of between about zero and 2° with the major fluxdirection of the silicon carbide growth species), but because the seedwafer is off-axis (i.e., the macroscopic seed surface is neitherperpendicular nor near-perpendicular to the crystallographic c-axis ofsilicon carbide) the seed surface possesses a higher density of surfacesteps per unit area compared to a seed with a macroscopic seed surfaceperpendicular to (or nearly perpendicular to) the crystallographicc-axis of silicon carbide. These surface steps act as nucleation sitesenabling a step-flow growth mode which is favorable for the nucleationand growth of high-quality silicon carbide.

The invention provides good polytype stability and stable growth afternucleation, the avoidance of split defects, and the resolution of thethermal stress component in the basal plane is dominated by the presenceof radial temperature gradients in the crystal which is favorable for alower probability of the thermal stress related formation of basal planedislocations in the silicon carbide crystal during growth or cooldown.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1. A seeded sublimation method of growing large single crystals of silicon carbide with reduced defects, the method comprising: nucleating growth on a seed crystal growth face that is between about 1° and 10° off-axis from the (0001) plane of the seed crystal; while establishing a thermal gradient between the seed crystal and a source composition that is substantially perpendicular to the basal plane of the off-axis crystal.
 2. A seeded sublimation method of growing large single crystals of silicon carbide with reduced defects, the method comprising: establishing a major growth temperature gradient between a silicon carbide seed crystal and a silicon carbide source composition; and positioning the seed crystal on the seed holder with the macroscopic growth face of the seed crystal at an angle of between about 70° and 89.5° with respect to the major growth gradient.
 3. A method according to claim 2 comprising positioning the seed crystal on the seed holder with the macroscopic growth face of the seed crystal at an angle of between about 70° and 80° with respect to the major growth gradient.
 4. A method according to claim 2 comprising establishing the major temperature gradient with a total temperature difference of between about 50 and 200° C.
 5. A method according to claim 2 comprising presenting the basal plane of the seed crystal as the growth surface.
 6. A method according to claim 2 comprising slicing the resulting bulk crystal into wafers
 7. A method according to claim 6 comprising polishing the sliced wafers.
 8. A method according to claim 6 comprising growing an epitaxial layer of a semiconductor material on the sliced wafer.
 9. A method according to claim 8 comprising growing an epitaxial layer selected from the group consisting of silicon carbide and Group III nitrides.
 10. A seeded sublimation method of growing large single crystals of silicon carbide with reduced defects, the method comprising: establishing a major growth temperature gradient between a silicon carbide seed crystal and a silicon carbide source composition; presenting a growth face on the seed crystal that is between about 10 and 10° off axis from the (0001) plane; and positioning the seed crystal on the seed holder with the growth face at an angle of between about 70° and 89.5° with respect to the major growth gradient.
 11. A method according to claim 10 comprising establishing the major growth gradient between silicon carbide source powder and the seed crystal.
 12. A method according to claim 10 comprising presenting the seed crystal on a seed holder that forms an angle of between about 70° and 89.5° with respect to the major growth gradient.
 13. A method according to claim 10 comprising presenting the seed crystal on a seed holder that forms an angle of between about 70° and 80° with respect to the major growth gradient.
 14. A method according to claim 10 comprising presenting the seed crystal on a seed holder that forms an angle of about 86° with respect to the major growth gradient.
 15. A method according to claim 10 comprising presenting a growth face on the seed crystal that is about 4° off-axis from the (0001) plane.
 16. A method according to claim 10 comprising presenting a seed crystal that has a polytype selected from the group consisting of the 4H and 6H polytypes of silicon carbide. 